Whiteflies of the genera Bemisia (sweet potato whitefly) and Trialeurodes (greenhouse whitefly) are major pests of crop plants throughout the world, causing economic losses especially due to the transmission of plant viruses during feeding (i.e. they act as ‘virus vectors’). Bemisia tabaci is capable of transmitting more than 60 different Geminiviridae plus a number of criniviruses, many of which belong to the Begomoviruses such as African cassava mosaic virus (ACMV), Bean golden mosaic virus (BGMV), Bean dwarf virus, Tomato yellow leaf curl virus (TYLCV), Tomato mottle virus (TMV), and others. Both tropical and temperate crops are affected, such as tomatoes, beans, cucurbits, potatoes, cotton, cassava and sweet potatoes. To date, the main control strategy is the application of insecticides, aimed at killing adults, juveniles and eggs. Besides the substantial costs of insecticide application this practice has a severe environmental impact. Moreover, B. tabaci is difficult to control with insecticides due to emerging resistance to the active ingredients.
In order to reduce insecticide application, there is a need for new ways of controlling whitefly-induced crop damage and losses, both in field-grown and greenhouse-grown crops. From literature it is known that volatile components can directly influence insect behaviour (e.g. Bruce et al., 2005, Trends Plant Sci. 10: 269-74). One way to control virus transmission by whiteflies is by identifying insect repellents, which can be applied on or near the crop plants, and/or insect attractants, which can be applied on nearby areas to lure the insect pests away from the crop. The problem in identifying attractants and/or repellents is that compounds that are known to attract one species may repel another species of insects. Often one cannot, therefore, draw conclusions about the attractant or repellent properties of compounds or compositions across species which may differ in their sensory perception and feeding behavior. Whiteflies, for example, investigate their host plants by labial dabbing (using mechanosensors and chemosensors) on the epidermal surface, before tapping into the vascular tissue (probing). Their decision at this point is influenced by e.g. constitutively produced repellents but probably also by properties of the leaf surface. Preference is directly related to performance and virus transmission, which occurs upon probing. In order to avoid virus transmission, probing should be prevented or at least reduced significantly. This means compounds that kill the whiteflies only after probing has occurred are not suitable as crop protection agents, as the virus will already have been transferred. In addition, insect predators of whiteflies should not be affected by the repellent or attractant, as these are useful in reducing the whitefly population.
Another problem in identifying suitable compounds and/or compositions for whitefly control lies in the fact that naturally occurring plant headspace compositions and the content of the glandular trichomes of plants contain a large number of different compounds in different concentrations, which vary between species and between individual plant lines or accessions within species. Even if a plant headspace composition as a whole is identified in having a certain effect on insect pests, identifying which components, or combinations of components, may be suitable as attractants or repellents is no easy task and to date there is no suitable repellent or attractant for whiteflies and other sap-sucking insect pests.
Zhang et al. (J. Econo Entomolog 2004, 97, p 1310-1318) tested 0.25% solutions of ginger oil as a repellent for B. argentifolii. In no-choice tests, only between 10.2 and 16.6% fewer adult whiteflies settled on the treated plants and no difference was found in the numbers of eggs laid on the plants. Increasing the concentration of ginger oil was associated with phytotoxicity, thereby preventing an effective use of ginger oil as whitefly repellent.
EP 0 583 774 describes the use of vegetable oil to reduce phytotoxicity of foliar insect control agents, whereby any type of insect control agent may be used.
Glandular trichomes are prominent on foliage and stems of the genus Lycopersicon (now classified as Solanum) and have been shown to produce a large number of secondary compounds, such as sesquiterpene hydrocarbons, sesquiterpene acids, methylketones and sugar esters. Several studies have tried to correlate the density of glandular trichomes with resistance against plant pests, such as maize earworm (Heliothis zea) or Colorado beetle (Kauffman and Kennedy, 1989, J Chem Ecol 15, 1919-1930; Antonious, 2001, J Environ Sci Health B 36, 835-848 and Antonious et al. 2005, J Environ Sci Health B 40: 619-631). Also the methylketones 2-undecanone and 2-tridecanone, stored in the glandular trichomes of L. hirsutum f. glabratum were shown to exhibit a toxic effect against fourth instar larvae of Colorado potato beetle and adult whiteflies B. tabaci, respectively (Antonious et al. 2005, J Environ Sci Health B 40: 619-631).
Antonious and Kochhar (J Environm Science and Health B, 2003, B38: 489-500) extracted and quantified zingiberene and curcumene from wild tomato accessions with the goal of selecting wild tomato accessions that can be used for the production of sesquiterpene hydrocarbons for natural insecticide production. However, whether such compounds are able to be used as whitefly repellents or attractants was not disclosed. It is mentioned that zingiberene has been associated with Colorado beetle resistance and beet armyworm resistance, while curcumene has been associated with insecticidal effects. The wild tomato species L. hirsutum f. typicum is mentioned to be resistant to B. argentifolii (Heinz et al. 1995, 88:1494-1502), but trichome based plant resistance could, of course, have various causes and from this paper one cannot make inferences regarding the presence or identity of compounds which have properties for attracting or repelling whiteflies.
Kostyukovsky et al. (Acta Horticulturae 2002, 576, 347-358) found that fumigants of essential plant oils (Cineole, safrole, essential oil from Labiatae or Foeniculum vulgare, or M-bromide) applied on pests of cut flowers (e.g. B. tabaci) at concentrations of 10-20 mg/l caused mortality after 2-4 hours of exposure (see Table 5).
Freitas et al. (Euphytica 2002, 127: 275-287) studied the genetic inheritance of the genes for the production of both the sesquiterpene zingiberene and glandular trichome types I, IV, VI and VII in interspecific crosses between L. esculentum (cultivated tomato, low in zingiberene) and wild L. hirsutum var. hirsutum (high in zingiberene). Zingiberene content in F2 plants contributed to B. argentifolii resistance by correlation and it was suggested to breed plants with simultaneously high levels of zingiberene, 2-tridecanone and/or acylsugars to contribute to higher levels of whitefly resistance. However, breeding for pest resistance is fundamentally different from developing pest repellent or attractant compositions. There is no indication as to the use of synthetic or purified zingiberene as whitefly repellent as such or in combination with other compounds.